A misfire condition in an internal combustion engine results from either a lack of combustion of the air/fuel mixture, sometimes called a total misfire, or an instability during combustion, sometimes referred to as a partial misfire. In such case, torque production attributable to the misfiring cylinder decreases, due to, among other things, a reduced level of combustion (i.e., manifested by a reduced Indicated Mean Effective Pressure (IMEP)). Additionally, un-combusted fuel enters the exhaust system, which is undesirable. Because of the possible impact on the ability to meet certain emission requirements, engine misfire detection is routinely provided on automotive vehicles. Most common approaches use various engine speed derivatives (e.g., crankshaft acceleration) to detect fluctuations attributable to one or more cylinders, and thus to detect misfire and to identify what cylinder or cylinders have misfired. Accordingly, most internal combustion engine systems already have such engine speed derivative data stored and available by virtue of the need to detect misfire.
While cylinder imbalance may be the result of misfire in a particular cylinder, there is also recognized an inherent cylinder-to-cylinder IMEP variation attributed to manufacturing and durability variations in the base engine and engine control hardware. Whatever the source, a level of cylinder imbalance can be measured by a so-called COVIMEP parameter (i.e., Covariance of Indicated Mean Effective Pressure), as seen by reference to co-pending U.S. application Ser. No. 11/973,099 filed Oct. 5, 2007 entitled “METHOD FOR DETERMINATION OF COVARIANCE OF INDICATED MEAN EFFECTIVE PRESSURE FROM CRANKSHAFT MISFIRE ACCELERATION”, assigned to the common assignee of the present invention and hereby incorporated by reference. U.S. application Ser. No. 11/973,099 in turn teaches a method for inferring COVIMEP from various misfire-originated engine speed derivatives. However, to effect improvement in the COVIMEP performance, it is desirable to identify which cylinder is the weakest (lowest IMEP) and which is the strongest (highest IMEP) so that one or more various control actions can be taken to reduce the variation or imbalance between the cylinders.
There is therefore a need for a system and method for low and high IMEP cylinder identification so as to allow for cylinder balancing.